This invention relates to anchor bolts used in ground excavation, and in particular, to an anchor bolt assembly for simultaneous support and excavation of weak ground.
When an excavation is made in weak ground, i.e., ground that cannot support itself for a reasonable period of time and for a reasonable size of excavation, stabilizing the excavation face, i.e., the front of the advancing excavation, becomes necessary. This is true for both surface and underground excavations.
The current practice addressing the problem of face stability may be discussed with reference to the two principal excavation approaches, i.e., conventional excavation and mechanized excavation.
In the conventional excavational approach, the following techniques are used:
(a) The entire face is divided into a number of smaller, self-supporting (or self-supporting with the assistance of a layer of sprayed concrete) faces for step-wise excavation (one smaller face at each step), and the excavation is advanced to a pre-determined location of the face. See FIGS. 1A and 1B where reference numerals 1 through 9 represent the smaller faces and 12 represents the entire face. FIG. 1A illustrates the initial excavation sequence with primary support. FIG. 1B represents the final sequence with final lining. The example shown in FIGS. 1A and 1B is for an approximate twenty-seven meter wide tunnel having a height of approximately eighteen meters. The principal shortcomings of this technique are that the advance is slow and there is an added expense of supporting the smaller faces and their peripheries.
(b) The face is pre-supported by using the technique of xe2x80x9cforepolingxe2x80x9d which comprises the installation of grouted anchors 10 (or dowels) normal to the face. See FIG. 2, illustrating a forepoled tunnel in coal, said tunnel having a roof 11, face 12, boreholes 13, seam 14, and resin 15 about the dowels 10. In the case of an underground excavation, the forepoles 10 are often installed at an upward-inclined angle at the crown of the opening. The disadvantages of this technique are that extra time is involved for the grout to harden; there is only the one-time use of the dowels; the hole for the dowel may be unstable; and the dowel only provides a xe2x80x9cpassivexe2x80x9d reinforcement (or stabilizing force). The passive reinforcement results from the outward (toward the excavated space) deformation of the ground which, in turn, is resisted by the dowel, thus producing tension in the dowel and, as a reaction, producing confinement (or stabilizing force) to the face. In contrast, the xe2x80x9cactivexe2x80x9d reinforcement is achieved by pre-tensioning the dowel (or bolt), which restricts the outward deformation of the ground.
(c) The face is supported by modifying the characteristics of the ground by means of jet grouting. This technique creates grouted columns (horizontally) which help to stabilize the face. The disadvantages of this technique are that: specialized equipment is needed, an exceptionally large amount of time is required for the operation, and the cost of the technique increases with depth because the jet grouting equipment has to be removed after each advance of the face.
In the mechanized excavation approach, a tunnel boring machine (TBM), normally a shielded TBM, is used. The following techniques are used for supporting the face:
(a) A physical shield is used to protect the workers and the equipment while the face is stabilized with compressed air or by ground freezing.
(b) A slurry or earth-pressure balance support is used at the face, ahead of the cutter head of the TBM.
The disadvantages of this technique include the questionable reliability of the technique, safety aspects, and slow advance of the excavation face. Furthermore, there is a large initial investment, inflexibility with regard to alignment of (tunnel) excavation, huge and expensive back-up system, and requirement of very skilled labor.
In view of the foregoing disadvantages inherent in the known types of devices now present in the prior art, the present invention provides both passive and active reinforcement, thus making it possible to control the outward deformation of the face, which is the critical aspect of ground control in design of excavations of weak ground.
The main objectives of the invention are to: provide continuous support and reinforcement to the front (or face) of an advancing excavation in weak ground; advance the face at a higher rate compared to the results of the current methods by performing simultaneous reinforcement and excavation; reduce the cost of advancing the excavation by eliminating the need for renewed reinforcement of the face after each advance; improve the reliability of the face reinforcement and, consequently, the safety of the workers; and eliminate the constraint of not exceeding a threshold for the radius of curvature of the tunnel axis, as in the case of excavating by a TBM.
To attain this, the present invention provides a multi-purpose anchor bolt, which performs two principal functions: (1) it acts as an active reinforcement for the ground when it is installed and tensioned, and (2) it is used to excavate and advance the face in steps. The invention helps to advance the face at a higher rate compared to the results of the current methods by performing simultaneous reinforcement and excavation functions. The invention reduces the cost of advancing the excavation by eliminating the need for installing new reinforcement at the face after each advance. The invention is a hybrid of traditional and mechanized (such as a full-faced TBM) excavation techniques.
These together with other objects of the invention, along with various features of novelty which characterize the invention, are pointed out with particularity in the claims annexed hereto and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and the specific objects attained by its uses, reference should be made to the accompanying drawings and descriptive matter in which there is illustrated a preferred embodiment of the invention.